The accompanying FIGS. 1 and 2 show examples of known valve actuating systems of the type described above. FIG. 1 corresponds to FIG. 3 of U.S. Pat. No. 6,941,910 which is incorporated herein by reference.
In FIG. 1, a single cam phaser (SCP) assembled camshaft 10 has two cams 12 and 14 that can be rotated relative to one another, the cam 12 being formed of two identical parts straddling the cam 14. Cam followers 18 and 20 in contact with the respective cams 12 and 14 are mounted on a common summation lever 16 which is pivotably supported by a pivot pin 22 on a lower rocker 24. At one end, the lower rocker 24 rests on a hydraulic lash adjuster 26 and at the other end it is in contact with the tip of the stem 28 of a valve 30. Up and down movement of the summation lever 16 causes the lower rocker 24 to pivot about the lash adjuster 26 in order to press down on the stem 28 of the valve 30, causing the valve to open by an amount proportional to the sum of the instantaneous lifts of the two cams 12 and 14.
FIGS. 2A, 2B and 2C are side, end and exploded perspective views, respectively, of a second known, and generally similar, valve actuating mechanism. To avoid repetition, like components have been allocated like reference numerals. The mechanism of FIG. 2 acts on the stems 28a and 28b of two valves 30a and 30b in unison and the summation lever 16 acts on two lower rockers 24a and 24b, resting as previously on lash adjusters 26a and 26b. A spring 34 acts upwards on the end of the summation lever 16 carrying the two cam followers 18.
It is inherent in such a valve actuating mechanism that there is a large clearance between the components when the cam followers 18 and 20 are on the base circles of both cams 12 and 14. Cam summation mechanisms using hydraulic lash adjusting elements consequently require some means for adjustment of the clearance to achieve two aims, namely:                to limit the expansion of the hydraulic lash adjusters, such that the correct amount of clearance is maintained in the system whilst the valves are closed, and        to hold the lower rockers in contact with the tips of the valves and with the clearance adjustment system by the expansion of the hydraulic lash adjusters, so as to force the clearance to occur between one of the cam profiles and its respective follower.        
To achieve these two aims, the actuating mechanism of FIG. 1 includes an adjustable stop plate 40 which acts to limit the movement of the pivot pin 22 connecting the summation lever 16 to the lower rocker 24. In the mechanism of FIG. 2, an appropriately graded shim 42 surrounding the pin 22 abuts the camshaft 10 to serve the same function.
While it would be possible to achieve the first of the two aims listed above by simply replacing the hydraulic lash adjusting elements with a mechanical clearance adjustment system, such a modification alone would result in the position of the lower rockers becoming indeterminate. This could lead to loss of contact with the valve tip or the rocker pivot, and potentially damaging impact forces between the components of the system.